Piezoelectric Micromachined Ultrasound Transducers (PMUTs) have emerged as a substitute to conventional ultrasonic sensors. A typical PMUT is a suspended membrane clamped at its edges and driven through piezoelectric effect by the application of an AC voltage. For instance, an air-coupled PMUT using Aluminum Nitride (AlN) as the active piezoelectric material is disclosed in Shelton, et al., “CMOS-Compatible AlN Piezoelectric Micromachined Ultrasonic Transducers,” 2009 IEEE International Ultrasonics Symposium (IUS), pp. 402-405, Rome, Italy, Sep. 20-23, 2009, incorporated by reference herein in its entirety. Other PMUTs have been demonstrated using, e.g., Lead Zirconate Titanate (PZT), which appears particularly promising in its Perovskite-phase due to a high degree of piezoelectric and ferroelectric coupling.
Thus, while a useful transducer may be micro-machined from Perovskite-phase PZT or other suitable material, many ultrasound applications require a range of operating frequencies that cannot be optimally addressed by a single transducer. For example, medical imaging may require frequencies ranging from 1-18 MHz (e.g., 1-6 MHz for deep structures and 7-18 MHz for superficial structures).
There remains a need for tunable PMUTs to address applications that have multiple operating frequencies or ranges of operating frequencies.